How do I set up a one-to-many relationship using Entity Framework Core?

Question

I'm working on a trucking API using Entity Framework (EF) Core. Basic CRUD operations are working fine using the repository pattern. There is an error in configurations I am implementing, however.

I want to obtain multiple trailers and trucks associated with single load, reflecting the one-to-many relationship.

public class LoadConfiguration : IEntityTypeConfiguration<Load>
{
    public void Configure(Microsoft.EntityFrameworkCore.Metadata.Builders.EntityTypeBuilder<Load> builder)
    {
        builder.Property(p=>p.Id).IsRequired();
        builder.HasOne(t=>t.Customer).WithMany().HasForeignKey(p=>p.CustomerId);
        builder.Property(p=>p.LoadedFrom).IsRequired();
        builder.HasMany(p=>p.Trailer).WithOne().HasForeignKey(t=>t.TrailerId); 
        builder.HasMany(p=>p.Truck).WithOne().HasForeignKey(t=>t.TruckId); 
        builder.Property(p=>p.Destination).IsRequired();
    }
}

public class Truck:BaseEntity
{
    public int PlateNo { get; set; }
    public string ModelName { get; set; }
    public Location StateCode { get; set; }
    public int PollutionCertificateValidity { get; set; }
    public int DateOfPurchase { get; set; }
    public int FitnessCertificateValidity { get; set; }
}

public class Load:BaseEntity
{
    public Customer Customer { get; set; }
    public int CustomerId { get; set; }
    public string LoadedFrom { get; set; }
    public Trailer Trailer { get; set; }
    public int TrailerId { get; set; }
    public Truck Truck { get; set; }
    public int TruckId { get; set; }
    public string Destination { get; set; }
}

public class Trailer:BaseEntity
{
    public int TrailerCapacity { get; set; }
    public Truck Truck { get; set; }
    public int TruckId { get; set; }
}

public class BaseEntity
{
    public int Id { get; set; }
}

Answer 1

A one-to-many relationship is defined by using navigation collections, that has the capacity to hold many Trucks and Trailers. You can choose the collection type freely, but I would suggest ICollection generic type.

Modify your Load class as follows:

public class Load:BaseEntity
{
    public Customer Customer { get; set; }
    public int CustomerId { get; set; }

    public string LoadedFrom { get; set; }

    public string Destination { get; set; }

    // navigation collections
    public ICollection<Trailer> Trailers { get; set; }
    public ICollection<Truck> Trucks { get; set; }
}

You will then be able to set up the relationship in your LoadConfiguration class by using the pluralized name:

builder.HasMany(p=>p.Trailers).WithOne();
builder.HasMany(p=>p.Trucks).WithOne();

.. even though EF Core will be smart enough to figure out the relation by convention so the fluent configuration is redundant.